S7-300 With MM440 Profibus Problem

Hi

I have some communication issues regarding a PLC and a couple of Drives. They are communicating on a Profibus network.

The PLC is a Siemens S7-300 (CPU314C-2 DP) Step-7
And 13 Drives Siemens MM440


Baud Rate = 1.5Mbps
Profile = Standard

Before the baud rate was setup to 12 Mbps and the profile was DP.

My problem is not consistent, it only seem to appear when the process is running. The connection is lost on a couple of my drive. The drive is faulting. I’m getting this error:



A070: No setpoints received from Profibus-DP communication board. Triggered by A702, A703, and A704. The telegram failure time set in P2040 has run out.

A702: The link to the profibus is interrupted.
A703: No setpoints or invalid setpoint are being received from the profibus master.
A704: At least one configured internode transmitter is not yet active, or has not has failed.

And its not always the same drive that is faulting. It could be one or it could be all of them. I did talk with the technical support guys from Siemens and they tried to help me out but I’m still getting the same problem.


The only way I can run profibus right now is by only having one drive on the network else its faulting.


I tried:
- To slow down the baud rate from 12 Mbps to 1.5 Mbps.
- Longer telegram failure time.
- Checked that all my setpoints were valid.
- Re-did the connection and connector.
- Re-did the grounding.
- Grounded with using the connector and tried also grounding outside the connector.
- Checked that my ground was in the specs.


I did not check the signal with a scope. I’m a little worried about EMC because the cables are running pretty close to the power (600V). But I’m at least 30 cm away from the power cables.

If you have any suggestion for me that would be really appreciated.

Thanks for your time.
Phil

Hi Phil

I my experience check these things again.
1) The bus terminating resistors (One cable in bus connector = resistor ON)
2) Profibus-cable groundings (shield grounding throught the Profibus connector IS NOT good enough).
3) If those doesn't fix up you problem, remove all bus connectors and measure loop resistance of the Profibus-cable. If one terminating resistor is ON -> resistance should be approximately 220 Ohms. This way you can eliminate the cable brakes and bad connectors.

Basics to good and reliable Profibus-network -> Good groundings!

Ok I will try these out pretty soon.

1) The bus-terminating resistor is on at the PLC and at the last drive.

2) The way it’s grounded right now it’s only in the connectors itself. And I was wondering also is it bad to take the shield and twisting it together and soldering it to a ground wire or do I really need to used a cable gland or clip. And should I ground at every station externally if the connector is not good enough.

Hi Again

In normal case, if you have several cabinets which including some Profibus-nodes -> Make groundins for the incoming and outgoing Profibus-cable, it is good enought. If all equipments are in same cabinet, "I use" two separate grounding points -> first beside the CPU and the other one, after cable middle point (where it is easy to do).

You don't have to twist or solder any cable shields, just strip the cable protection 2-4cm and connect this point to the groud (cabinet cover or separate bus bar). You can use cable gland, clips or even cable ties (last one, temporary of course) all of them works.

Amiko_Phil said:

Ok I will try these out pretty soon.

1) The bus-terminating resistor is on at the PLC and at the last drive.

2) The way it’s grounded right now it’s only in the connectors itself. And I was wondering also is it bad to take the shield and twisting it together and soldering it to a ground wire or do I really need to used a cable gland or clip. And should I ground at every station externally if the connector is not good enough.

Click to expand...

You must make sure the shield is properly connected in all the connectors.

The shield should automatically ground through the connector onto the backplane of the chassis of the PLC, but some third party devices don't do that, and I'd recommend you strip some of the insulation on the cable back by those devices and connect the shield to the panel's chassis.

You're communicating on standard DP I assume? In that case also make sure you're not overwriting your output peripherals with the wrong data.

You must also make sure the cable is daisy chained properly. I find that running from a tap box to the drive causes problems with the terminating.

That's a micromaster 440 drive right?

In that case a flashing orange or solid orange light on the drive's communication card means that is isn't on the bus. Flashing green means it is, but there is not intelligent command word, and solid green means that it is on the bus, and is receiving an intelligent command word. Check out that light when your drives misbehave.

This is a picture of one of the cabinet, there are three of these.


I know the cable look pretty ugly, but one of the tech guys from Siemens told me that the minimum length for the cable between each drive should be at least 1 meter long. But I'm not 100% sure that he is right.
And about the profibus module, i will take a look at that pretty soon.
Thanks for you help guys

Phil

Amiko_Phil said:

This is a picture of one of the cabinet, there are three of these.

I know the cable look pretty ugly, but one of the tech guys from Siemens told me that the minimum length for the cable between each drive should be at least 1 meter long. But I'm not 100% sure that he is right.
And about the profibus module, i will take a look at that pretty soon.
Thanks for you help guys

Phil

Click to expand...

That minimum length sounds right. I see you've got lots of orange lights.

If you suspect this is an installation issue, why not go to the Profibus website http://www.profibus.org and, from the 'Downloads' section there, get copies of the Installation Guidelines, and Cabling & Assembly Handbook. They're available as free pdf's and take all the doubts out of 'should it be grounded / shouldn't it be grounded; minimum/maximum lengths; minimum separation' etc issues. Once this has been nailed down (not literally!), if you still have problems, you can go back to Siemens and tell them it's their equipment, not your network.

Regards

Ken

I've never had a problem with 'minimum length' before but have had major issues with grounding.

Profibus needs to earthed (preferably with clamps) in as many places as possible and make sure the panel itself has a good earth.

I once experienced a really weird intermittent fault on a bank of drives that, in the end, was solved by reinforcing the earth bond to the panel.

Also, try the even lower bus speed of 187.5k.
It could be enough for you to run the drives.

What about the S7-314 ? Are there entries in the diagnostics buffer about Profibus problems ?
If so, you could try to set the bus profile to "user-defined" and increase the "Retry limit" from 1 to 2 or 3. This will make the Profibus less sensitive to errors.

Ken M said:

If you suspect this is an installation issue, why not go to the Profibus website http://www.profibus.org and, from the 'Downloads' section there, get copies of the Installation Guidelines, and Cabling & Assembly Handbook. They're available as free pdf's and take all the doubts out of 'should it be grounded / shouldn't it be grounded; minimum/maximum lengths; minimum separation' etc issues. Once this has been nailed down (not literally!), if you still have problems, you can go back to Siemens and tell them it's their equipment, not your network.

Regards

Ken

Click to expand...

I had a very similar problem with some MM440's and a 315 CPU last year.

I also thought I had checked everything, I eventually called Siemens and told them something was wrong with their hardware.

When they were there, I discovered I was writing the output block with zeros, and the writing the correct data in another place, so the drive didn't know what to do

So they won't let me forget that one.

Well the problem seems to be fixed now.

A guy from Siemens came in to help me with that problem.
And i was missing some OB's in my program.
He put in OB82, OB86, OB121 and OB122. And now i dont get
anymore faults.

Well thanks guy for the replys

Are you sure your problem is fixed ? from what you have described, you have now put in blocks that prevent the plc from stopping in the event of a drive falling off the bus. Check your diagnostic buffer - this will indicate if/when any stations are falling of the bus.

Amiko_Phil said:

Well the problem seems to be fixed now.

A guy from Siemens came in to help me with that problem.
And i was missing some OB's in my program.
He put in OB82, OB86, OB121 and OB122. And now i dont get
anymore faults.

Well thanks guy for the replys

Click to expand...

Was your PLC going into "Stop" mode? It would cause the drives to fault like that yes. But what is causing the "stop"?

Like mentioned above, check your diagnostic buffer